For each variable, determine whether it is best thought of as discrete or continuous.VariableDiscreteContinuous$($a$)$ The actual length of a roll of plastic wrap advertised to be $30$ feet long$($b$)$ The lifetime, in hours, of a $100-$watt light bulb manufactured by Bingley Corporation$($c$)$ The cranial capacity $($space inside the skull$)$ of a human skull$($d$)$ The highest grade level $(1$st$,2$nd$,$ or $3$rd$)$ completed by a high$-$school dropout